The invention relates to radio frequency communications and, more particularly, to a frequency division duplex (FDD)/time division duplex (TDD) system. and method for radio frequency communications over limited spectrum and for low costs.
Radio components are some of the most expensive parts of radio frequency (RF) communications equipment. This is particularly the case in cordless or wireless telephony. In RF communications and, particularly, in cordless or wireless telephony, costs and operational requirements can be very important to the success of communications equipment designs. In the engineering of those designs, the designer is often presented with design constraints imposed or dictated by costs of components or by operational requirements. Such costs and operational requirements are particularly important considerations when communications equipment is intended for lower-end consumers, such as individuals and households.
In RF communication technology, various standards established by the industry and other sources often dictate aspects of performance and equipment requirements. Standards have been established, for example, for cordless or wireless telephony products and other communications devices. Certain of the most common standards of the cordless telephone industry include the Cordless Telephony Second Generation (CT2) standard, the European Conference of Postal and Telecommunications Administrations (CEPT) standard, referred to as the Cordless Telephony First Generation (CT1) standard, the Cordless Telephony First Generation Plus (CT1+) standard, and the Digital European Cordless Telecommunications (DECT) standard, among others.
The CT2 standard, for example, employs a time division duplex (TDD) system and methodology. In TDD, transmit and receive communications occur among two stations, such as, for example, a handset and base set unit of a cordless telephone, in a burst manner at distinct intervals of time. In the past, devices conforming to CT2 have transmitted and received over an identical carrier frequency within the bandwidth dictated by the standard. Communications have been possible in TDD units because different time intervals are employed for transmissions and receptions by each station. During an interval that one station is transmitting, the other is receiving, and vice versa, all over the same bandwidth. Devices built according to the CT2 standard have been considered lower-end devices, that is, the devices are typically low-cost to consumers. This low cost is, in part, attributable to the use in those devices of only a single radio front end. That is possible in CT2 devices because communications occur over the same carrier frequency in the TDD manner. The prior TDD devices, however, at least those devices conforming to the CT2 standard, have implemented the TDD methodology using a single carrier frequency. It has previously been thought that use of limited bandwidth through implementation of TDD methodology over a single carrier frequency provides the greatest advantages. This has not necessarily resulted, however, in lowest cost for the prior TDD devices, as later detailed.
Other cordless telephone standards, such as the CT1 standard, at times have employed a frequency division duplex (FDD) concept. In typical FDD, transmit and receive communications occur over two distinct, separate carrier channels. Thus, two FDD communications stations, such as, for example, a handset unit and a base set unit of a cordless telephone, each transmit and receive over different carrier channels. While a first unit is transmitting over a particular channel, the second unit is receiving on that same channel. Then, when the second unit transmits, it does so on an entirely different channel, and the first unit receives on that different channel.
FDD systems have tended to be more expensive than TDD systems because additional radio front end components have been required in prior FDD systems in order to accomplish the transmissions and receptions over the separate channels.
Beyond TDD and FDD, other communications methodologies are continually being developed. Certain promising methodologies include use of a dual channel synthesizer and spread spectrum approaches. A dual channel synthesizer may be operable in the case of dual channel communications. Prior to employing dual channel synthesizers, communications across different channels required generation of an external local oscillator (LO). A drawback of that approach was that additional filters are required to attenuate the frequency image (sometimes referred to herein as "spurs`). The use of dual channel synthesizers overcomes that drawback. Such prior use incidentally increases costs, however, because of the expense for components of two synthesizers, rather than just one.
In spread spectrum technology, a sequential noise-like signal structure is employed to spread normally narrow band information signals over a relatively wide band of frequencies. The receiver in such systems correlates the signals to retrieve the original information signal. Spread spectrum technology can provide certain benefits. A disadvantage of spread spectrum technology, however, is increased expense for devices operating according to the technology because significant processing capacity is necessary.
As is apparent, there are numerous ideas and approaches to radio frequency communications and, in particular, to such communications in cordless telephony. On the one hand, TDD methods can be advantageous because of the minimal spectrum necessary for such communications. On the other hand, FDD methods provide advantages of continuous transmission and reception. The limitations of both such methods, though, are apparent, as previously discussed. The newer communications methods also provide certain advantages, but they do so only at greater expense or by use of more spectrum. It would, therefore, be a significant improvement in the art and technology to provide an improved system and method for radio frequency communication which is low cost and requires less spectrum.
The present invention employs a unique and novel system and method for FDD/TDD radio frequency communications over a limited spectrum and for limited costs in order to provide those advantages, as well as others. Though the background of the invention has been described, in part, with respect to cordless telephone applications and, in particular, to applications under the various standards and emerging technologies described above, the invention has other and varied applications which will be hereinafter more fully understood. The invention is, thus, a significant improvement and advance in the technology and art.